Ring laser gyroscopes utilizing counterpropagating laser beams are well known. These devices are used for measuring rotation and angular velocity of the ring laser gyroscope by combining portions of the counterpropagating modes or waves to generate a beat frequency representative of the differences in frequency between the opposing modes or waves. The term "mode" is used herein interchangeably with the word "wave", and means a resonant traveling wave of radiant energy propagating within a ring laser cavity. As the ring laser body is rotated about its sensing axis, the frequency of waves propagating in one direction within the cavity increases while the frequency of waves propagating in the opposite direction decreases. The difference in frequency between the counterpropagating modes creates a change to the beat frequency which is proportional to the rate of rotation about the sensing axis. By measuring the beat signal, the rate of rotation of the ring laser about its sensing axis is measured.
For the ring laser gyroscope to function at low rates of rotation, frequency locking or "lock-in" must be overcome. Frequency lock-in occurs when two oppositely traveling waves in a resonant cavity which should have slightly different frequencies combine in a single frequency standing wave. Thus, for low rates of rotation of the ring laser about its sensing axis, the frequencies of the waves lock together, and the gyroscope is insensitive to small rates of rotation. The effects of lock-in are described in detail in Laser Applications, edited by Monte Ross, Academic Press, Inc., New York, N.Y., 1971, in the article entitled, "The Laser Gyro," by Frederick Aronowitz, pages 133-200.
It is well known that the principal cause of lock-in is the mutual scattering of energy from each of the beams into the direction of the other. This mutual scattering, or backscatter, is explained in detail in Aronowitz, supra, pages 148-153. Briefly, the difference frequency between two counterpropagating waves in a ring laser is governed by the equation EQU .psi.=a+b sin .psi.
where .psi. is the instantaneous phase difference between the counterpropagating waves, a=k.OMEGA. where k is a proportionality constant and .OMEGA. is the rate of angular velocity of the ring laser about its sensing axis, and b is proportional to the magnitude of backscattered energy. Where a is smaller than b the beat frequency is equal to zero and the frequencies of the counterpropagating ring laser modes are the same. To have a gyroscope output signal which is a measure of rotational rate, .OMEGA., of the ring laser body, a must be greater than b.
One way of eliminating lock-in is to oscillate the ring laser body mechanically. By mechanically oscillating, or dithering, the laser structure, a rotation rate is superimposed on the gyroscope such that most of the time a is greater than b, and the effects of b are minimized or eliminated. A gyro employing mechanical dither is discussed in U.S. Pat. No. 4,115,004 entitled "Counterbalanced Oscillating Ring Laser Gyro," which issued Sept. 19, 1978 to Thomas J. Hutchings and Virgil E. Sanders and which is assigned to the same assignee as this patent.
Another method of minimizing the effects of lock-in is the directional dither of the magnetic field of a Faraday cell disposed within a ring laser path. Within the ring laser cavity, linearly polarized laser waves are converted to circularly polarized light whose vector rotates in the same direction as the windings in the Faraday cell. The circularly polarized light waves react with the magnetic field as they pass through the Faraday cell, and an effective increase or decrease in optical path length occurs, depending upon the direction of the field and the direction which the waves are traveling. After leaving the Faraday cell, the circularly polarized light is converted back to linearly polarized light. By oscillating the current in the Faraday cell windings, the magnetic field oscillates and varies the effective optical path lengths of the oppositely propagating waves in a nonreciprocal manner. Magnetic dithering using a Faraday cell is explained in Aronowitz, supra, pp. 157 through 159.
The above-described antilock-in techniques are passive, i.e., they are not dependent upon the active laser gain media. With those methods the effects produced on waves propagating in one direction in the laser path are equal and opposite to the effects produced on the waves traveling in the opposite direction.